The scriptures have ordained that if a star occurs on two different days in the same month, a birthday must be celebrated only on the second occurrence of that star. For birthdays and shraddha, we must follow the rules laid down by the scriptures.
For a given day, if the star under which one is born exits for 12 nazhigai-s from sunrise, the birthday is celebrated on that day; if not, it is celebrated the previous day.
For example: On a Wednesday if the star Revati exists for 12 nazhigai-s from sunrise, that day is considered to be a birthday. If Revati exists for 11 and ½ nazhigai-s on Wednesday, the birthday is celebrated on Tuesday.